The present invention relates to damping force control type shock absorbers capable of adjusting damping force characteristics.
Tube-type shock absorbers attached to suspension systems of vehicles, for example, automobiles, are generally arranged as follows. A piston connected with a piston rod is inserted in a cylinder having a hydraulic liquid sealed therein. In response to a stroke of the piston rod, the piston slidingly moves in the cylinder, causing a flow of hydraulic liquid. The hydraulic liquid flow is controlled to generate a damping force by a damping force generating mechanism comprising an orifice, a disk valve, etc. There is a publicly known damping force control type shock absorber capable of adjusting damping force characteristics by changing the flow resistance of the damping force generating mechanism using a variable orifice, and so forth.
Japanese Patent Application Publication No. 2009-281584, for example, discloses a damping force control type shock absorber having a check valve provided in a base valve in the bottom of a cylinder. The check valve allows the flow of hydraulic liquid from a reservoir to a bottom-side chamber in the cylinder. The shock absorber further has a check valve provided in a piston assembly to allow the flow of hydraulic liquid from the bottom-side chamber to a piston rod-side chamber in the cylinder. Further, the shock absorber has a damping passage provided outside the cylinder to communicate between the piston rod-side chamber in the cylinder and the reservoir. The damping passage is provided with a damping force control mechanism enabling adjustment of damping force by controlling the flow of hydraulic liquid.
In the above-described structure, during both the extension and compression strokes of the piston rod, the hydraulic liquid flows from the piston rod-side chamber in the cylinder to the reservoir through the damping passage, and damping force is generated by the damping force control mechanism. Thus, in response to either of the extension and compression strokes of the piston rod, a flow of hydraulic liquid occurs always in one direction in the damping passage. Therefore, for both the extension and compression strokes of the piston rod, damping force can be generated by a single damping force control mechanism to adjust the damping force characteristics.
The following problem is associated with a damping force control type shock absorber using a single damping force control mechanism to generate damping force against the flow of hydraulic liquid occurring in one direction in a damping passage for both the extension and compression strokes as disclosed in Japanese Patent Application Publication No. 2009-281584. That is, because both extension and compression damping force characteristics are adjusted by using a single damping force control mechanism, the damping force characteristics for either of the extension and compression strokes affect the damping force characteristics for the other stroke. Accordingly, the damping force characteristic control range is limited, so that it is difficult to obtain desired damping force characteristics.